This is a Magpul Industries Accessory known as the Ladder Rail Panel, model MAG013-BLK.
Ladder Panels offer very low-profile 1913 Picatinny rail coverage. The Santoprene material provide a rubbery surface for improved weapon control and rail protection. Easily cut for custom lengths" each panel fits one side of a carbine-length rail (18 slots). One panel per package. Manufactured by Falcon Industries" Inc.
Features:
Very low-profile 1913 picatinny rail coverage
Semi-rubbery material
Easily cut for custom lengths
Improves weapon control and rail protection
